The TIA/EIA Interim Standard, Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS-95 (Jul. 1993) defines a "Pilot Channel", also known as a "pilot", as an unmodulated, direct sequence spread spectrum signal transmitted continuously by each CDMA base station. The Pilot Channel allows a mobile station to acquire the timing of the Forward CDMA Channel, provides a phase reference for coherent demodulation, and provides a means for making signal strength comparisons between base stations for determining when to handoff. The pilot sequence is a Walsh code "0" spread by a pair of modified maximal length PN sequences with period 2.sup.15. A Traffic Channel is defined as a communication path between a mobile station and a base station that carries user and signaling traffic, and includes a Forward Traffic Channel and Reverse Traffic Channel pair. Different base stations are identified by different pilot channel PN sequence offsets.
Section 6.6.6.2.6 of IS-95 is concerned with the maintenance of the Active Set, Candidate Set, and Neighbor Set. The Active Set is the set of pilot channels associated with the CDMA Channels containing Forward Traffic Channels assigned to a particular mobile station. The Candidate Set is the set of pilot channels that have been received with sufficient strength by the mobile station to be successfully demodulated, but have not been placed in the Active Set by the base station. The Neighbor Set is the set of pilot channels associated with the CDMA channels that are probable candidates for handoff. Normally, the Neighbor set contains pilot channels associated with CDMA channels that cover geographical areas near the mobile station. Once a pilot channel has been selected into the Active Set, the Forward Traffic Channel and the Reverse Traffic Channel convey digital voice and/or data to and from the mobile station, respectively.
A Remaining Set is defined to be the set of all allowable pilot offsets, excluding the pilot offsets of the pilot channels in the Active Set, Candidate Set, and Neighbor Set.
Reference is made to FIG. 2 which is an example from the prior art standard illustrating how pilot channel strength is used to add or drop a pilot channel from a particular set. Further, FIG. 3 shows the prior art general flow of pilot channels from the Neighbor Set 51, to the Candidate Set 52, to the Active Set 53, including the possibility that a pilot channel may be grouped into the Remaining Set 54. The CDMA mobile station maintains the Active Set by tracking pilot strength, which is the ratio of received pilot channel energy to overall received energy. The overall goal is to keep the Traffic Channel uninterrupted.
If a pilot is in the Active Set, only the base station may command the mobile station to drop this pilot channel into one of the alternative sets. A turnaround time parameter may be defined as the time required for a weak pilot channel dropped from the Active Set into the Neighbor Set to be placed back into the Candidate Set, if at the moment it is dropped, its signal strength were to instantaneously increase above a specified threshold (T.sub.-- ADD). This threshold is specified in the IS-95 Standard as the signal strength level at which a pilot may be moved from the Neighbor Set into the Candidate Set.
A Handoff is defined by IS-95 as the act of transferring communication with a mobile station from one base station to another. A Hard Handoff is defined as a handoff characterized by a temporary disconnection of the Traffic Channel. Hard Handoffs occur when the mobile station is transferred between disjoint Active Sets, the CDMA frequency assignment changes, the frame offset changes, or the mobile station is directed from a CDMA Traffic Channel to an analog voice channel. A Soft Handoff is preferable and is characterized by commencing communications with a new base station on the same CDMA frequency assignment before terminating communications with the old base station.
There are instances when the conventional implementation of IS-95 is unsatisfactory. This is particularly true in weak signal and so called "pilot polluted" areas. Such areas include urban high-rise building areas, and areas of uneven terrain. It has been found that in these areas, a strong pilot channel may be dropped from the Active Set due to a temporary loss of signal strength, only for the dropped pilot channel to reappear shortly as a strong signal. This situation may reduce the quality of the communication link or even result in a dropped call.
Reference is made to U.S. Pat. No. 5,577,022 to Padovani et al., issued Nov. 19, 1996 and entitled "PILOT SIGNAL SEARCHING TECHNIQUE FOR A CELLULAR COMMUNICATIONS SYSTEM." As described in this patent, a mobile station maintains, in addition to the Active and Neighbor sets, a list of Candidate and Pre-Candidate Sets of pilot channels. Base station entries from Neighbor Set may be assigned to the Pre-Candidate and Candidate Sets, and eventually to the Active Set, based on an analysis of signal strength.